Methods for oxygen delignification and ozone bleaching of pulp

ABSTRACT

A method for treating pulp through oxygen delignification and ozone bleaching is disclosed. The method uses a modified oxygen recovery unit which separates a mixture of oxygen and ozone thereby generating a stream of ozone and a stream of oxygen; feeding the stream of oxygen to an oxygen delignification unit; feeding a pulp to the oxygen delignification unit; feeding the stream of ozone to an ozone bleaching unit; feeding the delignified pulp to the ozone bleaching unit; and recovering the delignified and bleached pulp.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority from U.S. ProvisionalApplication Ser. No. 62/268,641 filed on Dec. 17, 2015.

BACKGROUND OF THE INVENTION

The present invention relates to methods for delignification of pulpusing oxygen combined with ozone bleaching of the pulp.

Oxygen delignification is the step between digesting wood chips in pulpmaking and pulp bleaching operations. Oxygen delignification is designedto dislodge and disintegrate residual lignin left in the pulp after thedigestion step using oxygen and alkali. It is the direct extension todelignification that occurs during digestion. Contacting pulp in anaqueous alkaline medium with oxygen causes oxidation of lignin which notonly breaks molecules but also makes it water soluble. Oxidation ofcolor imparting groups reduces the Kappa Number. As a result, theconsumption of bleach chemicals in the bleach plant is reduced.Delignification with oxygen is a more gentle way of reducing the KappaNumber than by extended digesting and bleaching. In recent decades, newpulp mills have been increasingly adopting oxygen delignificationsystems as an advantageous step in reducing environmental impact andachieve a better return on economic investment.

The most commonly practiced oxygen delignification consists of thefollowing steps. The first step involves adding washed pulp into amixer, adding caustic, adding oxygen and steam to bring the temperatureto a range of 70° to 95° C. and introducing this pulp mixture into thebottom of a vertical tall reactor in a continuous fashion. The pulp willflow upwards while lignin in the reactor gets oxidized in the alkalinemedium thereby dissolving and disintegrating the lignin and dislodgingit from the pulp fibers. The reactor is maintained at 5 to 12 bargpressure to improve on oxygen uptake. The residence time for pulpflowing through a commercially practiced reactor is in the range ofbetween 20 and 100 minutes.

Oxygen delignification can be performed with both medium as well as highconsistency pulp. Due to limited effectiveness, difficulty in mixing ofoxygen and other operating problems with high consistency pulp, oxygendelignification has not achieved widespread success when compared withmedium consistency pulp.

Oxygen delignification works with pulps from both types of woods,hardwood and softwood, reducing the Kappa Number up to 35% and 50%respectively. In the case of hardwood, a two stage approach is neededwhere two reactors are placed in series. The first stage is maintainedat a higher pressure and lower temperature with less residence timewhile the second stage is usually maintained at lower pressure but athigher temperatures and greater residence times.

There has been limited success using a single stage high efficiencyreactor towards achieving short term delignification at smaller andexperimental scales. Consequently, there has not been any commercialapparatus embodying this concept.

Bleaching with ozone offers a more aggressive reaction with residuallignin and would be offered as a substitute technology versus bleachingusing bleaching chemicals like Chlorine, Chlorine Dioxide, Peroxide andothers. Ozone offers a very effective bleaching process which hasvirtually no residual impact on the environment. Chlorine and ChlorineDioxide produce adsorbable organic halides (or AOX) in wastewater;compounds that do not easily break down in nature and pose a risk toaquatic life.

The use of ozone for pulp bleaching is limited in the industry due toits relatively higher cost. Ozone is produced by reacting nearly-pureoxygen (or air) in a corona or plasma discharge which provides theenergy needed to combine diatomic oxygen (O₂) into the triatomic form(O₃) commonly referred to as ozone. In fact, ozone is never produced inits pure form and is always a minor component in a mixture whichincludes ozone at concentrations of 3 to 15% with the balance comprisingeither oxygen or air; depending on the method used to produce the ozone(oxygen or air feed).

For oxygen-fed ozone generators, the ozone gas mixture is a combinationof ozone, oxygen and a small amount of nitrogen. For ozone bleaching,ozone is the effective compound. Oxygen is not an effective bleachingagent; however it has value in other processes in the pulp mill; forexample, oxygen delignification.

There exists technology that employs adsorbents to separate ozone fromoxygen by adsorbing ozone and allowing the oxygen to pass through theadsorbent bed to be re-used as a feed-gas to the ozone generator. Suchtechnology offers significant cost savings to the operator as they canrecover a substantial (up to 75%) portion of oxygen which is a majorfactor in the cost of ozone production. Such systems might be referredto as oxygen recovery units

SUMMARY OF THE INVENTION

Pulp mills represent an industry where the need for ozone and oxygenoften co-exist in a local area within the production facility. Thisinvention provides a means of separating an ozone/oxygen gas stream toan ozone stream and an oxygen stream. Ozone will be adsorbed in anadsorbing bed while oxygen passes through the bed. The oxygen can beused for delignification of pulp in the fiber line following thedigester. The oxygen gas stream pressure can be increased using acommercially-available oxygen compressor to raise the pressure to 5 to12 Barg, depending on the requirements of the oxygen delignificationreactor(s).

The ozone stream can be utilized for pulp bleaching following itsdesorption of the adsorption bed. Desorption is carried out first byde-pressurizing the bed, then sweeping the ozone to the end using acarrier gas, typically air.

In a first embodiment of the invention there is disclosed a method fordelignifying and bleaching pulp comprising the steps:

a) Feeding a mixture of oxygen and ozone to an oxygen recovery unitthereby generating a stream of ozone and a stream of oxygen;

b) Compressing and feeding the stream of oxygen to an oxygendelignification unit;

c) Feeding a pulp mixture which comprises a pulp slurry and a liquor tothe oxygen delignification unit;

d) Feeding steam to the pulp mixture thereby raising its temperature;

e) Mixing the pulp mixture of pulp, liquor, steam and oxygen therebyenabling the oxygen to react with lignin and become mixed with theliquor;

f) Feeding the mixture to at least one washing step thereby removingdissolved lignin from the pulp mixture;

g) Feeding the stream of ozone to an ozone bleaching unit;

h) Feeding the delignified, washed pulp mixture to the ozone bleachingunit; and

i) Recovering the delignified and bleached pulp.

The pressure in step b) can range from 5 to 12 Barg.

During step c), the temperature of the pulp mixture will increase fromabout 90° up to 120° C.

The mixing in step d) can be as long as up to 100 minutes in length.

The at least one washing step in step f) can be two or more washingsteps.

The pulp can be selected from the group consisting of medium and highconsistency pulp. The liquor is selected from the group consisting ofwhite liquor, partially oxidized white liquor and completely oxidizedwhite liquor. Typically this pulp is about 10% consistency.

The concentration of oxygen in the stream of oxygen is greater than 80%.

The oxygen delignification unit is typically a reactor.

Typically, the amount of oxygen present in the pulp between 0.011 to0.026 tons of oxygen per one ton of pulp.

The mixture of oxygen and ozone is typically produced by an ozonegeneration unit. The resulting mixture of oxygen and ozone from theozone generation unit is about 10% ozone and 90% oxygen.

The oxygen recovery unit of step a) is typically a pressure swingadsorption system.

The ozone bleaching unit of step g) is typically a reactor.

Acid may be added to the pulp before the pulp enters the ozone bleachingunit in step h).

The ozone is produced using commercially-available ozone generationequipment; specifically those that use oxygen with purity greater than90% as a feed gas. For typical ozone generation, oxygen is passedthrough the water-cooled tubes of the heat exchanger. The resultingdilute ozone containing gas stream can be fed directly to the oxygenrecovery unit.

The ozone generation unit produces a mixture of ozone and oxygen basedupon an oxygen feed. This ozone and oxygen mixture is separated using anoxygen recovery unit-style pressure swing adsorption (PSA) system. Ahigh purity adsorbent material that does not significantly decomposeadsorbed ozone such as selected silica gel or high silica zeoliteadsorbents will concentrate the ozone allowing most of the unreactedoxygen to be recycled back to the ozone generation unit or used in otherapplications.

The separated oxygen stream will be fed to an oxygen delignificationunit. The oxygen delignification unit is typically a reactor for masstransfer.

The delignified pulp is then passed through a washing step which willremove dissolved solids from the pulp as well as soluble lignins presentin the delignified pulp. The dissolved solids can interfere with ableaching step.

After the washing step, the delignified pulp is fed to an ozonebleaching unit. The ozone bleaching unit uses a reactor to contact thepulp with ozone. The ozone source here will be a mixture of ozone andair derived from the oxygen recovery unit and contacted with the pulpuntil a satisfactory level of whiteness is achieved.

The bleached pulp is then recovered and fed to the next steps of thepapermaking process or bundled for storage or transport.

The use of the reactors will cause a significant reduction in Kappanumber over the oxygen delignification reactor when combining with ozonebleaching in the reactor. The reactors will provide mixing to oxidizethe pulp further to achieve the low Kappa numbers. The pulp mill canremove or reduce the amount of chlorine based bleaching it performs.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a schematic of an oxygen delignification and ozonebleaching process according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning to the figure, a schematic of an oxygen delignification andozone bleaching process in communication with an oxygen recovery unit isdescribed.

The pulp which is typically a medium or high consistency pulp is fedthrough line 1 to an oxygen delignification unit A. The pulp willoptimally be about 10% pulp slurry. This pulp feed will typically be ata temperature of about 90° to 120° C. and a pressure of 5 to 12 barg.The pulp solution is typically a caustic solution aided by the presenceof magnesium sulfate. A blow tank will usually be present to assist inpressure reduction and separation of the pulp slurry and dissolvedgases.

The oxygen delignification unit A will receive oxygen through line 4from the modified oxygen recovery unit C which will separate a mixtureof ozone and oxygen. Typically the oxygen is gaseous oxygen and ispresent between 0.011 to 0.026 tons of oxygen per one ton of pulp. Themodified oxygen recovery unit C will typically have an external line 2Awhich will typically utilize clean dry air to assist in desorbing thebeds present in the oxygen recovery unit C.

The modified oxygen recovery unit C will receive through line 2 amixture of ozone and oxygen from an ozone generator E. The mixture ofozone and oxygen is typically about 10% ozone and 90% oxygen.

The reactor will thoroughly mix the oxygen and the pulp and theresulting delignified pulp will be fed through line 3 to a series ofwashers step B where dissolved solids and soluble lignin is removedthrough line 5. The pulp in line 3 will have the same consistency aswhen fed through line 1 but be nearer to atmospheric pressure. The Kappareduction is also greater than 35%. The dissolved solids and solublelignins removed through line 5 form a weak black liquor. The delignifiedpulp will then be fed through line 7 to an ozone bleaching unit D. Theconsistency of the pulp slurry will remain the same at about 10% and thepressure remains at near atmospheric after unit B. Acid is typicallyadded to this stream prior to it reaching the ozone bleaching unit D andthis stream is raised in pressure through a pump to a range of 5 to 12barg.

The ozone bleaching unit D will also be a reactor which will thoroughlymix the ozone and delignified pulp. The ozone will be derived from themodified oxygen recovery unit C and will be fed through line 6 to theozone bleaching unit. The mixture is approximately 10 wt % ozone withthe remaining 90% being clean dry air. The reactor will ensure that theozone and the delignified pulp are thoroughly mixed to provide thedesired level of bleaching. The final bleached pulp may then berecovered through line 9 and sent for further processing in the pulpmill.

While this invention has been described with respect to particularembodiments thereof, it is apparent that numerous other forms andmodifications of the invention will be obvious to those skilled in theart. The appended claims in this invention generally should be construedto cover all such obvious forms and modifications which are within thetrue spirit and scope of the invention.

What we claim is:
 1. A method for delignifying and bleaching pulpcomprising the steps: a) Feeding a mixture of oxygen and ozone to anoxygen recovery unit thereby generating a stream of ozone and air and astream of oxygen; b) Compressing and feeding the stream of oxygen to anoxygen delignification unit; c) Feeding a pulp mixture which comprises apulp slurry and a liquor to the oxygen delignification unit; d) Feedingsteam to the pulp mixture thereby raising its temperature; e) Mixing thepulp mixture of pulp, liquor, steam and oxygen thereby enabling theoxygen to react with lignin and become water soluble and mixed with theliquor; f) Feeding the pulp mixture to at least one washing step therebyremoving dissolved lignin from the pulp mixture; g) Feeding the streamof air and ozone to an ozone bleaching unit; h) Feeding the delignified,washed pulp mixture of step f) to the ozone bleaching unit; and i)Recovering the delignified and bleached pulp.
 2. The method as claimedin claim 1 wherein the pressure in step b) ranges from 5 to 12 Barg. 3.The method as claimed in claim 1 wherein during step c), the temperatureof the pulp mixture increases from about 90° to 120° C.
 4. The method asclaimed in claim 1 wherein the mixing is up to 100 minutes in length. 5.The method as claimed in claim 1 wherein the at least one washing stepis two or more washing steps.
 6. The method as claimed in claim 1wherein the pulp is selected from the group consisting of medium andhigh consistency pulp.
 7. The method as claimed in claim 1 wherein theliquor is selected from the group consisting of white liquor, partiallyoxidized white liquor and completely oxidized white liquor.
 8. Themethod as claimed in claim 1 wherein the concentration of oxygen in thestream of oxygen is greater than 80%.
 9. The method as claimed in claim1 wherein the oxygen delignification unit is a reactor.
 10. The methodas claimed in claim 9 wherein the oxygen is present in the pulp between0.011 to 0.026 tons of oxygen per one ton of pulp.
 11. The method asclaimed in claim 1 wherein the mixture of oxygen and ozone is producedby an ozone generation unit.
 12. The method as claimed in claim 10wherein the mixture of oxygen and ozone is about 10% ozone and 90%oxygen.
 13. The method as claimed in claim 1 wherein the oxygen recoveryunit is a pressure swing adsorption system.
 14. The method as claimed inclaim 13 wherein the pressure swing adsorption system contains anadsorbent.
 15. The method as claimed in claim 14 wherein the adsorbentis selected from the group consisting of silica gel and high silicazeolite.
 16. The method as claimed in claim 1 wherein the ozonebleaching unit is a reactor.
 17. The method as claimed in claim 6wherein the pulp is about 10% consistency.
 18. The method as claimed inclaim 1 further comprising adding acid to the pulp before the pulpenters the ozone bleaching unit.